Method of actuating magnetic valves and circuit for carrying out said method

ABSTRACT

The specification discloses a control circuit for electromagnetic devices and a method of operating such devices in which a higher voltage is supplied for actuating the devices to pull the armatures thereof to attracted position, while a lower voltage is supplied to hold the armatures in attracted position. In the circuit the coils of the electromagnetic devices are connected across a line in series with a voltage-dropping resistor and a bypass around the voltage-dropping resistor is closed automatically when a switch pertaining to a coil to be energized is moved to coil-engaging position.

United States Patent Inventor Heinrich Dick Heidenheim, Germany Appl.No. 873,457 Filed Nov. 3, 1969 Patented Oct. 19, 1971 Assignee VoithGetriebe KG (Breny), Germany Priority Nov. 8, 1968 Germany P 18 07 748.0

METHOD OF ACTUATING MAGNETIC VALVES AND CIRCUIT FOR CARRYING OUT SAIDMETHOD 12 Claims, 2 Drawing Figs.

U.S. Cl 317/154, 307/141.4

Int. Cl H0lh 47/10 Field of Search 317/154,

[56] References Cited UNITED STATES PATENTS 2,788,473 4/1957Breckman..... 317/154X 3,088,053 4/1963 Gately 317/154 X 3,114.08312/1963 Winchel 317/154X 3,116,441 12/1963 Gieffers 317/1485 FOREIGNPATENTS 899,090 4/1959 Great Britain 317/154 Primary ExaminerDavidSmith, Jr. Assistant ExaminerWilliam J. Smith Att0meyWalter BeckerMETHOD OF ACTUATING MAGNETIC VALVES AND CIRCUIT FOR CARRYING OUT SAIDMETHOD The present invention relates to a method of actuating a magneticworking valve, solenoid, or the like, which is to be hold in workingposition for a long period of time and which is provided with a magneticcoil to be connected to a source of voltage for attracting the movablemagnetic core, and which is furthermore provided with an iron jacket,iron yoke, or the like, which is closed only in attracted condition ofthe movable magnetic core and is adapted to be short circuit the excitedmagnetic flux.

Magnets of this type have,. with regard to their copper windings, to bedesigned for a high output in view of the high current attractioninorder to prevent the current whichflows also in attracted conditionfrom unduly heating up the coil packet. The overheating is particularlydisturbing when a plurality of magnetic valves are provided in anelectrohydraulic control system and when the magnetic valves have toremain in their working positions for a long period of time. Valveswhich are in effective positions over a long period oftime are verylarge and heavy which fact represents a particular drawback when aplurality of such valves are employed and, for instance, with vehicletransmission controls because with vehicles the weight'and spacequestion is particularly important.

it is therefore, an object of the present invention to provide a controlmethod for magnetic valves which will permit the design of smallmagnetic valves, even if such valves have to remain in their effectiveposition over along period of time, while simultaneously avoiding thedanger of overheating the windings.

This object and other objectsand advantages of the invention will appearmore clearly from the following specification in connection with theaccompanying drawing, in which:

FIG. 1 illustrates a control circuit for the actuating current of twomagnetic valves which are adapted selectively to be moved into and outof their effective position while the current control is effected bymaking a magnetic coil effective.

FIG. 2 is a control circuit for the actuating current of a plurality of'magnetic valves which are adapted selectively to be made effective andineffective while the circuit control with four magnetic valves iscarried put by making a preceding coil ineffective.

The above-outlined object has been realized according to the presentinvention by connecting in a manner known as per se during the period ofattraction of the movable magnetic coil the coil first to a source ofhigh-attraction voltage (I percent)'for producing a high-attractingmagnetic flux in the still open yoke, and after the magnetic core hasbeen attracted, during the entire working period of the magnetic valveto connect the coil to a low-holding voltage for producing a holdingmagnetic flux in the closed yoke, said holding voltage preferablyamounting to about from Ste 20 percent of the attracting voltage.

For carrying out the method according, to the present invention, adirect current circuit is suggested in which the currentfeeding line tothe exciter coil of the magnetic valve there is provided an Ohm resistorwhich is adapted to be bridged by a bypass line and a switch within thebypass line which switch is electrically or electronically controlledand when in rest position is open. In the current -feeding line to theexciter coil of the magnetic valve there is furthermore provided apreferably likewise electrically or electronically controlled switchwhich is in rest position is open and which is adapted to turn on andturn off the magnetic valve. The arrangement further more comprises adelay circuit for the delayed opening of the switch which is arranged inparallel to the resistor and brings about a delay of said last-mentionedswitch when the main switch has been thrown in.

The delay circuit is expediently designed as a so-called R-C- member,and the switch is deligned as a transistorized threestage currentamplifier circuit in which the base emitter section of the entrancetransistor is in series with the R-C- member and in which the collectingemitter section of the exit transistor is parallel to the barrierresistor.

The circuit described-so far is suitable for the current control whenactuating-only one magnetic valve. If the actuating current is to becontrolled for aplurality of magnetic valves.

which are independent of each other, starting from the lastmentionedcircuit, there exist two'possibilities of distinguishing the controlwhich are predetermined by the respective installation which isto behydraulically controlled by the magnetic valves.

One possibility:.When a new magnetic valve is to be made effective,another magnetic valve has to be made ineffective (switching'over. fromone magnetic coil to another magnetic coil). Second possibility: When anew magnetic valve is made effective, no other magnetic valve is madeineffective-(mere throwing in of a magnetic coil). In case of anelectrohydraulic control of aplanetary gear transmission for vehicles,for in stance when changing the velocity, ordinarily a transmissionbrake or clutch is disengaged and instead thereof another brake orclutch is engaged, which means that in hydraulic control one valve hasto open and another valve has to close. When the transmission is shiftedto rearward drive, for instance a transmission clutch or brake whichnormally in rest position isclosedhas'to be opened hydraulically towhich end a magnetic valvehas to bernade effective.

When a multivalve control is involved with a corresponding shift-overoperation, the coils of the magnetic valves and, a respective pertainingactuating switch are all arranged in parallel to each other. The barrierswitch, the electronic switch (transistor)bridgingthe barrier resistor,and the delay control (R-C-member) are common to all-parallely arrangedcoils. For switching over from one magnetic valve to another, also whena' timewise overlapping of the velocity ranges is required,.according toa further development of the invention first the current. supply to onecoil is interrupted before the current supply tothe other coil isestablished. Due to the preceding elimination of one current consumer,.asmall short time potential increase within-the circuit is produced whichis taken advantage of for starting the current control. if no timewiseoverlapping is required, it is, in conformity with a further developmentof the invention, suggested to throw-in a smaller resistor parallel tothe switch contacts of that magnetic valve which is to remain effectivefor a longer period of time while the time constant of the R-C-member isdesigned for the required overlapping time.

If merely a valve is addedto the circuit, a short time potential dropoccurs within the circuit where during the switching off a small voltageincrease occurred. This signal is not suitable for putting through thecurrent control. For magnetic coils which are merely to be added to thecircuit, it is therefore according to a still further development of theinvention suggested that the increase in the potential at the coil whichhas been added itself taken advantage of for putting through the currentcontrol. To this end, between the contact at the coil side and theconductor between the condenser and the resistor of the R-C-member,there is inserted a small condenser. This condenser places the increasein potential when the throwing in the coil to the entrance of input sideof the current amplifier circuit which in its turn closes the circuit.

Referring now to the drawing in detail, the general character of thecircuits of both figures is now being described since basically they arethe same. in these figures, the coils of the magnetic valves to beactuated are represented by the black rectangular symbol of aninductivity. Thus they symbolizc the magnetic valves and the latter ifthey are thrown in represent a certain condition of the installationwhich is being controlled by the electrohydraulic control system ofwhich only its electric part and the latter only partly is illustrated.in order to distinguish the individual magnetic valves in the drawings,the magnetic valves have been designated with the reference numerals lto 7. The circuit is shown for direct current, and all coils have oneterminal connected to the minus pole of the direct current voltagesupply furnished by the battery B, and B, respectively, which means tothe mass M. Through the plus pole P the coils are supplied with currentvia the bridgeable resistor R and R, respectively. As is customary,sparkaextinguishing diodes 8 to 14 are arranged in parallel to thecoils.

In conformity with the present invention, the barrier resistor R R whichis bridged only during the attraction period of the magnetic valve is sodesigned that the voltage drop along said barrier resistor amountspreferably to about from 78 to 93 percent of the full voltage of thedirect current network. The voltage which is available in thrown-incondition at both ends will then amount only to from 7 to 22 percent ofthe full network voltage. This means that in the thrown-in conditiononly approximately from 0.5 to percent of that electric output isconsumed in the magnetic valve which is consumed during the attraction(full voltage) because in the calculation of the power the voltageenters with the power two (N=U /R). A safe operation ,is also obtainedwhen the arrangement is subjected to considerable shocks-danger oftearing off of the electromagnetically lifted magnetic core-when themagnetic core is held fast by 2 percent of the attraction force, inother words the barrier resistor is designed for a voltage drop ofapproximately 86 percent. Only one such barrier resistor is required inthe control circuit even if a plurality of magnetic valves are to becontrolled with regard to the attraction current, because even whenadditional magnetic valves are thrown in, a short current surge will notdo any damage in view of the already thrown-in magnetic valves.

That terminal of the barrier resistor R,,, or R, which faces away fromthe plus pole is connected to a point 5,, S respectively where duringnormal operation prevails the reduced voltage, or the full networkvoltage when a magnet is just attracting. By means of this conductor,through the intervention of switches 15-18, the coils of the respectiverequired magnetic valves are connected to the current supply ordisconnected therefrom. The entire circuit can be connected anddisconnected through the main switch 19, respectively.

The further structure of the circuit is no longer the same for bothillustrated circuits, and for this reason the circuit according to FIG.1 and its operation will now be explained.

The current control circuit according to FIG. 1 comprises primarily thebridging circuit including the transistor T and the relay R,, and thedelaying R-C-member composed of the condenser C C and the resistor REach coil 1 and 2 has respectively associated therewith a condenser CC,.

The bridging switch 21 of the relay is open when in rest position. If bymeans of the switch 15 the positive connection of one of the coils l and2 receives the potential of the switch point 8,, first a relatively highcharging current passes through the pertaining condenser and alsothrough the base emitter section of the transistor which immediatelyturns on, i.e. becomes conductive in its collector-emitter section. Inthis way a delay of microseconds only occurs after the actuation of theswitch is before the full network voltage prevails at the coil of therelay R,, and after a delay of a few milliseconds until the relay hasexerted its attracting force, the full network voltage also prevails atthe magnetic valve coil which has been turned on together with theswitch 15. The magnetic core of the turned on valve is attracted.Simultaneously, the pertaining condenser is charged. The chargingvoltage equals the network voltage minus the voltage reduced by theresistor R The magnitude of the resistance and of the capacity determinethe charging period. With increasing saturation of the condenser, itscharging current decreases. If this charging current drops below thethreshold value of the transistor necessary for switching through, thebase emitter section of the transistor does not again become conductive,and the relay R drops off. The time constant of the R-C-member isdesigned for the attraction period of the magnetic valves (in themagnitude of 50 ms.), which means that the charging time of thecondenser is somewhat larger than its attraction period. Only when themagnetic valve has definitely attracted, is the relay allowed to dropand the energization output is permitted to drop to the fraction stillnecessary for holding when the iron yoke is closed.

When switching over, the current supply to the magnetic coil energizedup to this point is interrupted and its magnetic core drops off. Theoperation of the coil energized instead of the coil energized before orfor an additionally energized coil and the control of the currenttherefor will take place precisely in the manner described above.

The current control according to the circuit of FIG. 2 is initiated in amanner which is fundamentally different from that described inconnection with FIG. 1. When designing the circuit of FIG. 2, thedesigner started with the knowledge that with installations to becontrolled electrohydraulically, an oil flow is made effective orinefi'ective while another oil flow is made ineffective or effectiverespectively. Such an arrangement is normally the rule for controlinstallations for planetary gear transmissions of passenger cars. Suchtransmissions are controlled by hydraulically operable friction clutchesor brakes and, more specifically, in such a way that when effecting avelocity change, one clutch or brake is opened while the other isclosed, or one is closed while the other one is opened. It may beassumed, for instance, that when the magnetic valves pertaining to thecoils 3 and 6 have attracted (illustrated position), the first'velocityrange of a transmission is made effective. When shifting the switch 17from the right toward the left, the oil flow controlled by the magneticvalve 6 is turned off and instead the oil flow controlled by the valve 5is released. Due to this change, a shift-over from the first to thesecond velocity range is effected. When the switch 16 is fully shiftedfrom the left toward the right, a switchover is effected from the secondto the third velocity range. Starting from the switch position shown inFIG. 1 for the assumed first velocity range, for instance, by anadditional energization of coil 7 the rearward velocity range of thetransmission may be made effective.

The special feature in connection with the circuit of FIG. 2 is seen inthe way in which the current control is initiated when shifting thevalves. Even when a timewise overlapping of the working periods of themagnetic valves is necessary for a switch-over operation, first the coilof one magnetic valve is made currentless before the other will beconnected. This operation is effected by switch-over means in a normalmanner.

By switching off a current consumer, the potential at the switchingpoint S will increase. This minor increase in the voltage will cause acharging current for the condenser C the resistor R the base emittersection of the transistor T and the resistor R In view of this flow ofelectrons via the base emitter section of the transistor T thetransistor is switched through which means that its collector-emittersection becomes conductive. In this way the base emitter section of thetransistor T will through resistor R, be connected to the network. Arelatively strong current will flow whereby the collector-emittersection of this transistor will become conductive. Thus, through theresistor R the base emitter section of the transistor T is connected tothe network whereby its collector-emitter section will become conductiveand short circuit the resistor R This three-step transistor circuit isnecessary in order to be able by means of such low currents (fractionsof a milliampere) as represented for instance by the charging currentdue to a potential increase in the switching point 5,, to switch thehigh-attraction currents (about 20 amperes) which are required forattracting the magnetic cores. The right-hand portion of FIG. 2designated with the reference numeral 25 illustrates so to speak acurrent-amplifying circuit with a high amplifying factor. The successiveswitching through of the individual transistors takes place with anunbelievable speed. As soon as one of the contacts of switches 16 and 17opens, also the resistor R is bridged by the transistor T designed asoutput transistor. At this moment, the full network voltage prevails atswitching point 2 and at one side of the condenser C Due to this renewedincrease in the potential, all of the transistors will become fullyeffective in case they should not have switched through by the firstsmall increase in the potential. Thus, a circuit arrangement is involvedin which a minute small increase in the potential on the strip Sinitiates an avalanchelike and very fast increase in the switchingthrough action on the transistor T,. This bridging lasts as long as inview of the initiated increase in the potential a sufficiently highcharging current flows via the condenser C;,. In the meantime, theoppositely located contact of the reversing switch 16 or 17 must havebeen reached. The charging time for the condenser C, is bycorrespondingly dimensioning its capacity and the magnitude of theresistors R, and R so designed that a charging current flows at least aslong as the newly energized magnetic valve has attracted. The chargingcurrent decreases with increasing charge in an exponential way.Somewhere the charging current drops below the minimum valve which isnecessary for switching through the transistor T which means that thecharging current near the end of the charging period drops below thethreshold value of the transistor responsive current. This moment or thetime limited thereby is of interest for the invention, and this certaintime is in this connection to indicate the charging time for thecondenser. When the charging current drops below this threshold value,each of the transistors will become nonconductive in a manner analogousto the manner described above in connection with the successiveswitching through of the three transistors T T and T The bridging of theresistor R,,,, is thus eliminated at the end of the charging time forthe condenser C,. A drop in the potential occurs at the strip S, whichdrop similar to the response of the transistor row initiates aself-amplifying switching off of the electronic switch. A slight drop ofthe responsive threshold of the first transistor T initiates aspontaneous switching off of the resistor bridging. In order to be surethat the condenser C and the condenser C" (referred to further below)will in the intervals quickly discharge, there is provided a rectifierdiode 23.

In the discussions so far the resistor R, and the condenser C., have notbeen mentioned. These circuit elements do not affect the function of thebridging circuit '25 of the three transistors but merely serve for atimewise overlapping of the tuming-on periods when shifting from coil 3to coil 4. For purposes of explaining such an intended overlapping ofthe velocity ranges, it may be mentioned that the response of thebridging circuit with the three transistors is considerably faster thanthe dropping off of the magnetic core in the magnetic valve.

Before, following the opening of the left contact of the switch 16, themovable core of the magnetic valve 3 moves away from the remaining fixediron mantle to any material extent so that a larger airgap fonns, theresistor R,,, has already been bridged, and at the strip 2 thereprevails the full network voltage. This means that when the resistor R,has been so dimensioned as to its magnitude as the resistor R the coil 3will be passed through by a current corresponding to the holdingenergization in spite of the turning off by means of switch 16, and themagnetic valve will remain in its previous working position as long asthe resistor R is bridged. When this bridging is interrupted, tworesistors will be in the currentfeeding line to the coil 3. In this waythe voltage drop to the coil 3 becomes so high that the remainingvoltage will permit only such small energizing current to pass throughthe coil 3 that the magnetic filed created thereby will not be able anylonger to hold the magnetic valve so that the latter will return to itsrest position. The time up to the point when the bridging of theresistor R, is again interrupted when the switch 16 is switched from theleft to the right, i.e. until the magnetic valve of coil 3 returns toits rest position, is controlled by the magnitude of the condenser C...This time corresponds to the switching on overlapping time of themagnetic valves when changing from coil 3 to coil 4. At the time atwhich the righthand contact of the switch closes, the condenser C. willbe electrically parallel to the condenser C As a result thereof, thetime constant of the R-C-member is increased. By correspondinglydimensioning the condenser C it is possible so to extend the commoncharging time of the two condensers of the new R-C-member that theoverlapping time is obtained.

Principally, the condenser C could be eliminated and the condenser Ccould from the very start be made so large that its charging timecorresponds to the overlapping time. This, however, has the slightdrawback that with each change from one magnetic valve to anothermagnetic valve, the transistor T and naturally also the transistors T,and T, are under load for an unduly long time. If the switchingoperations are carried out particularly frequently, the relativeswitching on period of the transistors may become so high that in theswitching intervals of the transistors, the latter will not be able anylonger to cool off to the ambient temperature and might heat up to anonpermissible extent. The intermediate switching on period is in viewof the condenser C, reduced to the extent which is necessary under allcircumstances because only during the changeover when an overlapping isrequired, namely when changing over from coil 3 to coil 4, does theswitching non period of the transistor become relatively long, forinstance 0.3 seconds, and otherwise is short, for instance 20milliseconds.

The circuit according to FIG. 2 also shows a coil 7 pertaining to amagnetic valve which coil is energized without a previous turning off ofanother current consumer. in order nevertheless to be able to cause aslight current to flow through the bases emitter section of the entrancetransistor T,,, which slight current will initiate the bridging circuit,there is provided a condenser C, which, when the switch 18 is closed, iselectrically parallel to the condenser C The condenser C, has a capacitywhich is by a plurality of orders of magnitude smaller than the capacityof the condenser C In order to make sure that the charging current willactually flow through the base emitter section of the entrancetransistor and not through the condenser C a rectifier diode 26 isprovided which blocks toward the condenser C and is arranged between theterminal 24 of the condenser C, and the condenser C Following theclosure of switch 18, a low-charging current flows which in a mannerknown per se initiates the action of the bridging circuit 25. When thecoil 7 is by means of the switch 18 again separated from the currentsupply, an increase in the potential occurs which will initiate thebridging circuit 25 and thus will briefly overenergize the remainingenergized coils. This, however, will do no harm.

As will be evident from the above, the present invention solves anenergy problem which is closely related to space and weight. It is wellknown that magnetic valves are to be dimensioned more or less strongdepending on the required relative duration during which they have to beeffective so as to meet the heating up which increases with the lengthof time during which such magnetic valves are in action. A liftingmagnet for a 15 percent relative duration of action is considerablysmaller, lighter and less expensive than a lifting magnet of the samestrength for a percent relative duration of action. These differences inmagnitude are purely thermally required. Decisive is not the duration ofits active period but the electric energy introduced into the liftingmagnet per time unit. The present invention has drawn the conclusion ofthis finding and accordingly the coil is actuated only during the briefmoment during which a strong excitation of the magnetic flux isnecessary in a shocklike manner with a high power. In the remainingtime, the energizing current or the power is reduced to the extentwhich, with the now closed iron yoke of the lifting magnet, is necessaryfor holding the core. lf magnetic valves are employed, 2 percent of theattraction force will suffice. This means that a magnetic valvecontrolled as to the exciter current in conformity with the presentinvention with a lOO-percent active duration as seen from a timestandpoint has to be only as large as a magnetic valve actuatedaccording to standard methods and need be designed only for a 2-percenteffective duration because the power input and the heating up will inboth instances be the same. In this way, with the higher necessaryactive periods, reductions in the overall volume and in the price andweight are possible to from one-tenth to onetwentieth with respect tomagnetic valves of heretofore known control circuits.

The advantages realized by the present invention are seen in that themagnetic coils provided for high-medium time periods of active duty canbe designed as small, light and inexpensive as magnetic coils forextremely low-medium active duty while the number of elements isrelatively low and while such elements can be inexpensive and massproduced. The expenses for the circuits are less than the pricedifference of a magnetic valve for a lOO-percent relative active duty ofstandard operation IOO-percent relative active duty means when in onecycle the inertia temperature is reached) in contrast to an operationfor only -percent active duty which means that the expenses for thecircuit pay for themselves even if only one single magnetic valve isprovided, aside from space and weight advantages. The saving isconsiderable in connection with electrohydraulic control installationsor with other installations employing lifting magnets in which aplurality of lifting magnets are provided. A saving can also be realizedwhen the relative duration during which the magnetic valves areeffective is only short, for instance when it amounts only to 10percent, because the input of power with the operation according to theinvention is still less, and the coils may when taking advantage of thepresent invention also thermally be designed smaller than with thecustomary way of operation even if the valves are made effective onlyfor an effective duration of 10 percent. It is, of course, to beunderstood that the present invention is, by no means, limited to theparticular showing in the drawing but also comprises many modificationswithin the scope of the appended claims.

lclaim:

1. in a direct current system for controlling the supply of energy tothe coil of electromagnetic means having moveable armature means, asupply of direct current voltage, a voltagedropping resistor in serieswith said coil means across said supply, a normally open line bypassingsaid voltage-dropping resistor, a voltage sensitive control element insaid line responsive to a supply of voltage thereto to bring aboutclosing of said line, switch means between said voltage droppingresistor and said coil means adapted to be actuated between opened andclosed positions, circuit means including delay elements in the form ofa first resistor and a first capacitor in series therewith operativelyconnecting said switch means to said control element and operable inresponse to actuation of said switch means to supply voltage to saidcontrol element for a predetermined period of time commencing with theactuation of said switch means, said period of time being determined bythe value of said delay elements, said control element having aconnection to the said first resistor near the end thereof opposite saidfirst capacitor.

2. A direct current system according to claim 1, in which said controlelement includes transistor means including at least one transistorhaving the base thereof connected to the said resistor near the endthereof opposite to the capacitor.

3. A direct current system according to claim 2, in which saidtransistor means further includes a power transistor having itscollector-emitter connected in parallel with said voltage-droppingresistor, and an amplifying transistor having its collector-emittercircuit connected to the base of said power transistor and having itsbase connected to the collectoremitter circuit of said one transistor.

4. A direct current system according to claim 3, in which said coilmeans comprise the coils of a plurality of magnetic valves for amultispeed transmission, each coil having one end connected to thenegative side of said source and the other end connected to a respectiveswitch contact, a bus bar, switchblades connected to the bus bar andadapted to close on said contacts, said voltage-dropping resistorconnecting said bus bar to the positive side of said source, said firstcapacitor and first resistor being connected in series in the ordernamed between said bus bar and the negative side of said source, a firstPNP transistor having the base thereof connected to the said resistornear the end thereof opposite to the capacitor, a

power transistor having its collector-emitter path connected betweensaid bus bar and the positive side of said source, and

an amplifying transistor connected between the collectoremltter path ofsaid first transistor and the base of said power transistor and operableto make the latter conductive when said first transistor goesconductive.

5. A direct current system according to claim 4, in which, during atransmission shifting operation, at least one coil has the currentsupply thereto interrupted and thereafter at least one other coil hasthe current supply thereto established.

6. A direct current system according to claim 4, in which the timeconstant of the serially arranged first capacitor and first resistor isselected to provide a delay corresponding to the time required toattract an armature to attracted position.

7. A direct current system according to claim 4, which includes afurther resistor between said bus bar and the said contact pertaining toone of said coils, said further resistor having about the same ohmicresistance as said voltage-dropping resistor, and the time constant ofsaid serially arranged first capacitor and first resistor being selectedto provide a delay greater than the time required to attract an armatureto attracted position.

8. A direct current system according to claim 4, which includes afurther resistor between said bus bar and the said contact pertaining toone of said coils, said further resistor having about the same ohmicresistance as said voltagedropping resistor, a second contact on whichthe switchblade pertaining to the last-mentioned contact is adapted toclose, and a further capacitor connected between said second contact andthe resistor side of said first capacitor, the time constant of saidfirst capacitor and first resistor alone providing for a delay nogreater than the time for an armature to move to attracted position andthe time constant thereof when said switchblade is closed on said secondcontact so as to connect said further capacitor in circuit with saidfirst capacitor providing for a delay greater than the time for anarmature to move to attracted position.

9. A direct current system according to claim 8, which includes afurther coil having a respectively said contact and a further coilhaving a respective said contact and a respective switchblade connectedto said bus bar and closeable on the contact, a third capacitorconnected between the last-mentioned contact and said first resistornear the capacitor end thereof, and a diode poled toward said firstresistor and inter posed between said first capacitor and the connectingpoint of said third resistor to the capacitor end of said firstresistor.

10. A direct current system according to claim 4, which includes a diodeconnected from the negative side of said source to the resistor side ofsaid first capacitor and poled toward said first capacitor.

11. A circuit for controlling the supply of energy to the coil ofelectromagnetic means having a moveable armature; a direct currentvoltage source, a first resistor and a first switch in series with saidcoil across said source, a second switch actuatable to bypass said firstresistor, said circuit also comprising a delay circuit for controllingsaid second switch, said .delay circuit comprising a condenser and asecond resistor in series therewith, a transistor having thecollector-emitter path connected in controlling relation to said secondswitch, said capacitor and said second resistor and the base emitterpath of said transistor forming a series circuit connected in parallelwith said coil, said transistor being operable upon conduction of saidcollector-emitter path to actuate said second switch.

12. A circuit according to claim 11 in which said transistor is theinput stage of a current amplifier circuit, said current amplifiercircuit also including an output transistor having its collector-emitterpath connected in parallel with said first resistor, an amplifiertransistor having the collector-emitter path connected to the base ofsaid output transistor and having a base connected to thecollector-emitter path of said first mentioned transistor.

1. In a direct current system for controlling the supply of energy tothe coil of electromagnetic means having moveable armature means, asupply of direct current voltage, a voltagedropping resistor in serieswith said coil means across said supply, a normally open line bypassingsaid voltage-dropping resistor, a voltage sensitive control element insaid line responsive to a supply of voltage thereto to bring aboutclosing of said line, switch means between said voltage droppingresistor and said coil means adapted to be actuated between opened andclosed positions, circuit means including delay elements in the form ofa first resistor and a first capacitor in series therewith operativelyconnecting said switch means to said control element and operable inresponse to actuation of said switch means to supply voltage to saidcontrol element for a predetermined period of time commencing with theactuation of said switch means, said period of time being determined bythe value of said delay elements, said control element having aconnection to the said first resistor near the end thereof opposite saidfirst capacitor.
 2. A direct current system according to claim 1, inwhich said control element includes transistor means including at leastone transistor having the base thereof connected to the said resistornear the end thereof opposite to the capacitor.
 3. A direct currentsystem according to claim 2, in which said transistor means furtherincludes a power transistor having its collector-emitter connected inparallel with said voltage-dropping resistor, and an amplifyingtransistor having its collector-emitter circuit connected to the base ofsaid power transistor and having its base connected to thecollector-emitter circuit of said one transistor.
 4. A direct currentsystem according to claim 3, in which said coil means comprise the coilsof a plurality of magnetic valves for a multispeed transmission, eachcoil having one end connected to the negative side of said source andthe other end connected to a respective switch contact, a bus bar,switchblades connected to the bus bar and adapted to close on saidcontacts, said voltage-droPping resistor connecting said bus bar to thepositive side of said source, said first capacitor and first resistorbeing connected in series in the order named between said bus bar andthe negative side of said source, a first PNP transistor having the basethereof connected to the said resistor near the end thereof opposite tothe capacitor, a power transistor having its collector-emitter pathconnected between said bus bar and the positive side of said source, andan amplifying transistor connected between the collector-emitter path ofsaid first transistor and the base of said power transistor and operableto make the latter conductive when said first transistor goesconductive.
 5. A direct current system according to claim 4, in which,during a transmission shifting operation, at least one coil has thecurrent supply thereto interrupted and thereafter at least one othercoil has the current supply thereto established.
 6. A direct currentsystem according to claim 4, in which the time constant of the seriallyarranged first capacitor and first resistor is selected to provide adelay corresponding to the time required to attract an armature toattracted position.
 7. A direct current system according to claim 4,which includes a further resistor between said bus bar and the saidcontact pertaining to one of said coils, said further resistor havingabout the same ohmic resistance as said voltage-dropping resistor, andthe time constant of said serially arranged first capacitor and firstresistor being selected to provide a delay greater than the timerequired to attract an armature to attracted position.
 8. A directcurrent system according to claim 4, which includes a further resistorbetween said bus bar and the said contact pertaining to one of saidcoils, said further resistor having about the same ohmic resistance assaid voltage-dropping resistor, a second contact on which theswitchblade pertaining to the last-mentioned contact is adapted toclose, and a further capacitor connected between said second contact andthe resistor side of said first capacitor, the time constant of saidfirst capacitor and first resistor alone providing for a delay nogreater than the time for an armature to move to attracted position andthe time constant thereof when said switchblade is closed on said secondcontact so as to connect said further capacitor in circuit with saidfirst capacitor providing for a delay greater than the time for anarmature to move to attracted position.
 9. A direct current systemaccording to claim 8, which includes a further coil having arespectively said contact and a further coil having a respective saidcontact and a respective switchblade connected to said bus bar andcloseable on the contact, a third capacitor connected between thelast-mentioned contact and said first resistor near the capacitor endthereof, and a diode poled toward said first resistor and interposedbetween said first capacitor and the connecting point of said thirdresistor to the capacitor end of said first resistor.
 10. A directcurrent system according to claim 4, which includes a diode connectedfrom the negative side of said source to the resistor side of said firstcapacitor and poled toward said first capacitor.
 11. A circuit forcontrolling the supply of energy to the coil of electromagnetic meanshaving a moveable armature; a direct current voltage source, a firstresistor and a first switch in series with said coil across said source,a second switch actuatable to bypass said first resistor, said circuitalso comprising a delay circuit for controlling said second switch, saiddelay circuit comprising a condenser and a second resistor in seriestherewith, a transistor having the collector-emitter path connected incontrolling relation to said second switch, said capacitor and saidsecond resistor and the base emitter path of said transistor forming aseries circuit connected in parallel with said coil, said transistorbeing operable upon conduction of Said collector-emitter path to actuatesaid second switch.
 12. A circuit according to claim 11 in which saidtransistor is the input stage of a current amplifier circuit, saidcurrent amplifier circuit also including an output transistor having itscollector-emitter path connected in parallel with said first resistor,an amplifier transistor having the collector-emitter path connected tothe base of said output transistor and having a base connected to thecollector-emitter path of said first mentioned transistor.